This invention relates to the electrical measurement art, and more particularly to a new and improved apparatus for detection and measurement of electrostatic quantities such as electrostatic fields, electrostatic voltages or electrostatic charges.
Current art electrostatic surface potential monitoring systems employ an electrostatic detector electrode surface which is vibrated in physical relationship to a test surface under measurement to produce a modulated signal, at the frequency of vibration, which is indicative of the test surface potential. In systems achieving high accuracy, which is independent of the detector to test surface separation distance, the detector signal developed is processed by demodulation and integration to produce a feedback signal which is fed back as a reference level for the detector. Using this well known feedback technique, such as disclosed in U.S. Pat. Nos. 3,852,667, 4,205,267, 4,370,616 and others, high accuracy is achieved by driving the voltage difference and therefore the electrostatic field between the detector surface and measurement surface to zero, thus making the measurement accuracy independent of spacing between these two surfaces.
However, a major disadvantage of this voltage/field nulling technique lies with the necessity of producing a feedback voltage level which is the same magnitude as the measured unknown. In electrophotographic applications, a measured surface voltage level of 2 to 3 kilovolts often is required. The requirement of producing high voltage at high speed to follow measured surface voltage variations places a cost restraint in high volume applications for electrostatic surface voltage monitors, such as direct employment in copy machines, or high speed printers, as well as presenting a safety hazard due to high voltage levels in the equipment. In addition, the use of hybrid circuits is impossible in circuitry carrying 2 to 3 kilovolts, thus nullifying the use of this cost reducing manufacturing technique.